1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly to a half-transmission type liquid crystal display device having functions of a light-transmission type liquid crystal display device and a light-reflection type liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device is generally comprised of two substrates and liquid crystal sandwiched between the two substrates, in which an intensity of electric field to be applied to the liquid crystal is controlled to thereby control a degree at which backlight passes through the liquid crystal.
A vertical-alignment type liquid crystal display device can completely shut out a light when no electric field is applied thereto. Namely, since a luminance in off-condition in a normally black mode is quite low, a vertical-alignment type liquid crystal display device can present a high contrast ratio in comparison with a conventional twisted nematic type liquid crystal display device.
In general, backlight consumes 50% or more among power consumed in a liquid crystal display device. Hence, a portable communication device is often designed to include a light-reflection type liquid crystal display device which includes a light-reflector in place of a backlight source for displaying images only by incident lights.
However, a light-reflection type liquid crystal display device is accompanied with a problem that displayed images cannot be seen when it is dark around the device.
As a solution to the problem, there has been suggested a half-transmission type liquid crystal display device including a light-reflection area and a light-transmission area, as a liquid crystal display device having advantages of both of a light-reflection type liquid crystal display device and a light-transmission type liquid crystal display device. For instance, Japanese Patent No. 2955277 has suggested such a half-transmission type liquid crystal display device.
FIG. 1 is a cross-sectional view of a first example of a conventional half-transmission type liquid crystal display device.
A half-transmission type liquid crystal display device 100 illustrated in FIG. 1 is comprised of a first substrate 101, a second substrate 102, and a liquid crystal layer 103 sandwiched between the first and second substrates 101 and 102.
The second substrate 102 is comprised of a second electrically insulating transparent substrate 104, an opposing electrode 105 composed of ITO (indium tin oxide) formed on the second transparent substrate 104 in facing relation to the liquid crystal layer 103, an alignment film 106 formed on the opposing electrode 105, an optical compensator 107 formed on the second transparent substrate 104 in opposite side with respect to the liquid crystal layer 103, and a polarizer 108 formed on the optic compensator 107.
The half-transmission type liquid crystal display device 100 is designed to have a first area 120 in which a light is reflected and a second area 121 through which a light passes. A structure of the first substrate 101 in the first area 120 is different from a structure of the first substrate 101 in the second area 121.
In the first area 120, the first substrate 101 is comprised of a first electrically insulating transparent substrate 109, a passivation film 110 formed on the first transparent film 109 in facing relation to the liquid crystal layer 103, a pixel electrode 111 composed of ITO and formed on the passivation film 110, a dielectric layer 112 formed on the pixel electrode 111 and having a wavy surface, a pixel electrode 113 covering the dielectric layer 112 therewith in wavy configuration and composed of aluminum, an alignment film 114 covering the pixel electrode 113 therewith, an optical compensator 115 formed on the first transparent substrate 109 in opposite side with respect to the liquid crystal layer 103, and a polarizer 116 formed on the optic compensator 115.
In the second area 121, the first substrate 101 is comprised of a first electrically insulating transparent substrate 109, a passivation film 110 formed on the first transparent film 109 in facing relation to the liquid crystal layer 103, a pixel electrode 111 composed of ITO and formed on the passivation film 110, an alignment film 114 formed on the pixel electrode 111, an optical compensator 115 formed on the first transparent substrate 109 in opposite side with respect to the liquid crystal layer 103, and a polarizer 116 formed on the optic compensator 115.
In the half-transmission type liquid crystal display device 100, liquid crystal molecules constituting the liquid crystal layer 103 are aligned so that major axes of them are perpendicular to the first and second substrates 101 and 102 when no electric field is applied to the liquid crystal display device 100. The liquid crystal molecules have negative dielectric anisotropy.
FIG. 2 is a cross-sectional view of a second example of a conventional half-transmission type liquid crystal display device.
A half-transmission type liquid crystal display device 150 illustrated in FIG. 2 is different from the half-transmission type liquid crystal display device 100 illustrated in FIG. 1 in a structure of the first substrate 101 in the first area 120.
That is, in the half-transmission type liquid crystal display device 150, the pixel electrode 113 composed of aluminum is covered with the pixel electrode 111 composed of ITO, and the alignment film 114 is formed on the pixel electrode 111. Except this difference, the half transmission type liquid crystal display device 150 is identical in structure to the half-transmission type liquid crystal display device 100.
The half-transmission type liquid crystal display device 100 illustrated in FIG. 1 displays images as follows.
In the first area 120, an external light enters the half-transmission type liquid crystal display device 100, and is reflected at the pixel electrode 113 acting as a reflector. Then, the reflected light passes through the liquid crystal layer 103 and the second substrate 102, and reaches a viewer.
In the second area 121, a backlight emitted from a backlight source (not illustrated) arranged below the first transparent substrate 109 passes through the first substrate 101, the liquid crystal layer 103 and the second substrate 102, and reaches a viewer.
As mentioned above, whereas an incident light reciprocates the liquid crystal layer 103 in the first area 120, an incident light passes through the liquid crystal layer 103 only in one-way in the second area 121, resulting in an optical path difference in the liquid crystal layer 103. In order to avoid such an optical path difference, a cell gap Dr of liquid crystal in the first area 120 is designed to be about half of a cell gap Df of liquid crystal in the second area 121, thereby optimizing an intensity of an output light caused by a difference in retardation between the first and second areas 120 and 121.
For instance, the cell gaps Dr and Df are designed equal to 2 μm and 4 μm, respectively.
The half-transmission type liquid crystal display device 150 illustrated in FIG. 2 displays images in the same way as the half-transmission type liquid crystal display device 100.
In order to make use of advantages provided by the above-mentioned half-transmission type liquid crystal display device and vertical-alignment type liquid crystal display device, Japanese Patent Application Publications Nos. 2000-29010 and 2000-35570 suggest a liquid crystal display device having function of both of half-transmission type and vertical-alignment type liquid crystal display devices.
A half transmission type liquid crystal display device having the first and second areas unavoidably has the cell gaps Dr and Df different from each other, in order to avoid the above-mentioned optical path difference in the liquid crystal layer 103.
However, the cell gaps Dr and Df different from each other cause a problem that liquid crystal molecules are inclined in non-uniform directions at a boundary between the first and second areas and in the vicinity of the boundary when electric field is applied to the liquid crystal layer, resulting in deterioration in visibility and reduction in a response speed.
Japanese Patent No. 2565639, based on U.S. patent application Ser. No. 879,256 filed on Apr. 30, 1992, has suggested a liquid crystal display device including a common electrode formed on a substrate. The common electrode is formed in alignment with a display area with a patterned opening for dividing the display area into a plurality of liquid crystal domains, and covers the substrate therewith in an area other than the opening.
Japanese Patent Application Publication No. 2000-250056 has suggested a liquid crystal display device including a pixel electrode formed with an opening in the form of a slit and in parallel with an orientation of alignment of liquid crystal molecules.
Japanese Patent Application Publication No. 2002-107724 has suggested a liquid crystal display device including a λ/4 double-refraction layer arranged between a light-reflection layer and a liquid crystal layer to thereby equalize a thickness of the liquid crystal layer in a light-reflection area to a thickness of the liquid crystal layer in a light-transmission area.
Japanese Patent Application Publication No. 2002-98951 has suggested a half-transmission type liquid crystal display device including a reflection electrode having a patterned opening having a side which is not in parallel with any sides of an effective frame of a liquid crystal display panel and any sides of a pixel pattern.